Large-capacity power-generating nuclear fission reactor plants normally have several hundred sealed tubular containers for housing fissionable fuel. To facilitate periodic refueling, which commonly is performed by replacing fractional portions of the total fuel at intervals and rearranging other fractional portions, these fuel rods or pins are conventionally assembled into bundles or groups of elements which can be manipulated as a single composite unit.
The fuel rods of each bundle are held mutually parallel and spaced apart by mechanical means. A typical fuel bundle comprises, for example, an 8.times.8 or 9.times.9 array of spaced fuel rods. Each fuel rod is usually more than 10 ft long, e.g., 14 ft, and approximately 1/2 inch in diameter.
To inhibit the fuel rods from bowing and vibrating due to high heat and high velocity of the coolant flowing past, the fuel rods are maintained in their spaced-apart relation by a plurality of spacers positioned at intervals along their length. Typical spacers for fuel rods comprise a lattice having a plurality of openings arranged in the designated pattern for spacing the parallel aligned fuel rods. The assembled bundle of a group of spaced-apart, parallel aligned fuel rods additionally each have their ends supported in corresponding sockets of upper and lower tie plates.
The typical fuel bundle assembly also comprises an open-ended tubular channel of suitable cross section, such as square, which surrounds the fuel rods. The fuel channel directs the flow of coolant longitudinally along the surface of the fuel rods and channels the neutron-absorbing fission control blades, which reciprocate longitudinally intermediate a 2.times.2 array of channeled fuel bundle assemblies.
A bail or handle is connected to the upper tie plate. When a hoist is coupled to the bail, the fuel bundle assembly can be lifted and transported as a unit. When supported by a hoist, the fuel bundle assembly hangs in a generally vertical position.
New fuel bundle assemblies are conventionally stored in racks installed in a new fuel storage pool. During an outage, spent fuel is removed from the reactor and replaced with new fuel. The new fuel storage pool has a depth which is sufficient for storage of new fuel, but insufficient for storage of spent fuel just removed from the reactor. The spent fuel coming from the reactor must be transferred to a deep pool as soon as it leaves the reactor. To accomplish this, fuel coming from the reactor is moved to the transfer pool where it is moved by the carriage of the fuel transport system to the spent fuel storage pool. At this time the carriage must return to the pickup area to accept another fuel bundle.
During a conventional refueling operation of the reactor core, about 25% or more of spent or burned fuel bundles within the reactor core are replaced with fresh fuel bundles. An upper pool of water is typically located above the reactor core for providing, for example, shielding of radiation from the fuel bundles, and a conventional bridge or gantry is movable over the pool for refueling the reactor core. The bridge includes a trolley-mounted grapple which is telescopically extended downwardly through the pool and into the reactor core to grab one of the fuel bundles by its handle at the too thereof, and is then retracted upwardly to remove the fuel bundle. The fuel bundle is continuously maintained underwater to provide shielding thereof as well as for allowing water to flow upwardly through the fuel bundle to cool it. This prevents overheating due to the continuation of nuclear reactions therein which occur at a level which is substantially reduced relative to that occurring in an operating reactor core.
In a one-bridge refueling system, each spent fuel bundle is removed from the core, translated horizontally through the upper pool to an adjacent fuel storage pool and placed vertically in one of a horizontal array of storage racks. The spent fuel bundles are stored for up to several years until such time when the spent fuel is to be relocated to a long-term storage site. A fresh fuel bundle is then transported by the bridge from the fuel storage pool back to the reactor core and positioned therein. Since a typical reactor includes several hundred fuel bundles, a substantial amount of time is required to remove the spent fuel bundles and replace them with fresh fuel bundles.
Furthermore, in a single-bridge system, conventional fuel shipping casks, which may weigh up to about 100 tons, must be individually lifted into the fuel storage pool so that the spent fuel bundles may be inserted therein. The lifting of such a heavy cask entails a risk that the cask may drop and consequently damage the pool and/or the fuel bundles.
Another type of refueling system used in most PWRs and some BWRs uses two bridges with a transfer machine therebetween. One bridge carries individual fresh or spent fuel bundles between the reactor core and the transfer machine, and the other bridge transports individual spent or fresh fuel bundles between the transfer machine and the fuel storage pool. The transfer machine then conveys the spent and fresh fuel bundles between the two bridges. In this way, an entire refueling operation may be carried out in less time than is needed when using a single bridge since the two bridges and transfer machine may be synchronized while being operated contemporaneously, with each transport device carrying a respective fuel bundle. This system also eliminates the risk of dropping a shipping cask into the storage pool since one of the bridges may be used for transporting fuel between separate pools containing the stored fuel and the shipping casks.
In this two-bridge system, the two bridges are typically located in separate buildings, one containing the reactor core and another containing the fuel storage pool. The common wall between the two buildings must necessarily provide a seal for radiation and pressure between the two buildings and therefore requires a relatively complex transfer tube extending therebetween through which fuel bundles are transferred. The transfer tube is typically oriented either horizontally or at an incline so that the passage between the two buildings is relatively small, thereby reducing the complexity of the required seals therebetween. It is desirable to transport the fuel bundles primarily in an upright position as they are moved laterally or sideways through the respective pools so that water may continually flow vertically upwardly therethrough for cooling the fuel bundles. The fuel bundles must, therefore, necessarily be temporarily upended or moved from their vertical position to an inclined or horizontal position for passage through the transfer tube. The conventional transfer machine therefore provides an upending device at one end of the transfer tube to initially rotate a vertical fuel bundle to the required horizontal position for passage through the transfer tube, and another upending device at the other end thereof to return the fuel bundle to the preferred vertical position. Suitable seals are provided at both ends of the transfer tube to prevent leakage of the water therethrough.
When the fuel bundles are positioned horizontally, natural convection cooling by the water being channeled therethrough is reduced since the vertical path therethrough has been reduced or eliminated. Accordingly, the fuel bundle must be transported relatively quickly through the transfer tube to reduce the likelihood of overheating of the fuel bundles, and additional procedures must be established to provide effective cooling in the event of any failure of the transfer machine while the fuel bundles are upended.
When control blades are also removed, the process changes slightly. Since the control blades are not as radioactive as the fuel, the control blades can be stored temporarily in the new fuel storage pool. This is accomplished using racks especially designed to store control blades. When all the fuel leaving the reactor has been moved to the spent fuel storage pool, the control blades can be moved.
A fuel transfer system is disclosed in copending U.S. patent application Ser. No. 07/834,947 to Townsend et al. entitled "Fuel Transfer System", wherein fuel bundles may be continuously maintained in an upright position during the entire refueling operation. That fuel transfer system includes a transfer pool containing water at a level above the reactor core. A fuel transfer machine installed underwater in the transfer pool includes a carriage for transporting fuel bundles. The carriage is selectively movable through the water in the transfer pool and individual fuel bundles are carried while being held upright on the carriage. A first movable bridge is disposed over an upper pool containing the reactor core and a second movable bridge is disposed over a fuel storage pool, with the transfer pool being disposed therebetween. A fuel bundle may be moved by the first bridge from the reactor core and loaded into the carriage, which transports the fuel bundle to the second bridge, which in turn picks up the fuel bundle and carries it to the fuel storage pool. However, the carriages of the fuel transfer system of Townsend et al. lack the capacity to carry both fuel bundles and control blades.